For example, a silicone oil, having a high flash point and a high level of safety compared with a mineral oil, may be used as an insulating oil in an on-board (e.g., railway) oil-immersed electric apparatus (e.g., transformer) with an emphasis on safety.
For an oil-immersed electric apparatus using a mineral oil as an insulating oil, a conventionally known method diagnoses an internal fault such as abnormal overheating using, for example, the types or concentrations of gas components of an insulating oil or the concentration ratios between a plurality of gas components of an insulating oil as indices, without stopping the apparatus.
For example, a known method divides a diagnosis map, where a horizontal axis represents a ratio between two gas components and a vertical axis represents a ratio between the other two gas components, into areas and then performs an abnormal diagnosis based on which area of the diagnosis map a plot corresponding to a measured value is placed in (for example, see Japanese Patent Laying-Open No. 2004-200348 (PTD 1)).